# Increasing spin-photon interface efficiencies via a tunable Fabry-Perot microcavity

**Abstract:** The development of a quantum internet crucially relies on quantum nodes combining a robust storage of quantum states, high-fidelity processing of quantum information and an efficient interface to photons. Seminal proof-of-principle experiments have established the nitrogen-vacancy (NV) center in diamond as one of the prime candidates for the implementation of a network node. However, the entanglement rates between remote NV centers are limited to tens of Hertz due to the small fraction of coherent photon emission. A promising strategy to overcome this limitation is to couple NV centers to a photonic resonator while preserving long optical and spin coherence times.

To this end, we developed a miniaturized tunable Fabry-Perot cavity platform in which we integrate a high-quality micrometer-scale diamond membrane. We deterministically couple individual NV centers and show a Purcell enhancement of the coherent photon emission by a factor of 35. The fraction of coherent emission is thereby increased from ~3% to ~46%. In combination with an improved photon collection efficiency this approach is poised to enhance the NV-NV entanglement rates by more than three orders of magnitude.

We push the limits of our platform by embedding charge-tunable, self-assembled InAs quantum dots into our microcavity. An epitaxially grown semiconductor mirror combined with a novel gating design and surface passivation technique facilitates cavity Q-factors of up to Q-factors of up to 10^6. For optimally coupled quantum dots the exciton-photon coupling rate g exceeds both the photon decay rate κ and exciton decay rate γ by a large margin (g/γ = 14, g/κ = 5.3). We observe pronounced vacuum Rabi oscillations in the time domain, photon blockade at a one-photon resonance, and highly bunched photon statistics at a two-photon resonance.

**Series:**IQIM Postdoctoral and Graduate Student Seminar Series